Last month investigators backed by armed policemen raided Media-Most's offices, reinforcing criticism that President Putin intended to crack down on news outlets critical of his administration.

In the US, White House spokesman Joe Lockhart said: " We are quite concerned about some of the steps taken against the free media."

Sergei Parkhomenko, editor of the Gusinsky-owned "Itogi" weekly magazine, said that Mr Gusinsky was being held in Moscow's 18th century Butyrsky jail, not the more modern Lefortovo prison where high-ranking detainees are usually held.

"It is the ugliest jail in Moscow, where only criminals are held," he said.